maintaining heat in the chamber

So we cooked Thanksgiving Fri instead of Thurs, due to scheduling issue. Anyway, I decided to try a twice smoked ham. Honey mustard then brown sugar, and then onto the OK Joe I just got. Apple/Oak mix for some good smoke, and the cooking chamber around 220F. I smoked it for two hours, then foiled it and used pineapple juice and more sugar for a glaze. Here's the issue I had. I was able to keep the cooking chamber at 220F-240F, but was having issues getting the IT up to anywhere near 140. How do y'all heat your smoker, charcoal, wood, or what. I would like to attempt a Boston Butt if I can figure out how to keep that thing hot enough.

This was a 13 lb. ham, not frozen, allowed to sit about an hour before I started. I suspect that the IT thermometer might be the culprit, since it was all I had around the house. I will probably have to invest in a better thermometer.

Even if you don't spring for one those fancy digital wireless units, a regular meat thermometer or instant read digital pen style meat thermometers are only $10 or $12. Every cook should have at least one. I have two and use them on everything.